1. Field of the Invention
This invention relates generally to the field of microwave transmission and more particularly to a means of aligning the phase of signals at radiating elements of a series fed antenna.
2. Description of the Prior Art
Transmission lines that carry electromagnetic signals have a given, fixed length, thus, a signal of a given frequency will have a certain phase or degrees per unit length associated with it. As the frequency of the transmitted signal increases, the number of degrees increases as well for a given length of time. Therefore, at specific locations along the transmission line the phase angle at these locations will change relative to one other when the frequency of the signal is changed. This relative phase difference at transmission line locations is detrimental for some electromagnetic signal applications.
One application in which relative phase angle differences at predetermined locations is a particular drawback is in the field of array antennas. There are two common types of array antennas, a series fed antenna and a corporate fed antenna. A corporate fed antenna generally has one input and the signal from this input passes through a divider which feeds all the radiating elements approximately in phase. Another type of array antenna is a series fed antenna which is generally one long transmission line having the radiating elements spaced along the line in succession. In the case of a corporate fed antenna, as the frequency of the signal is changed the phase at each radiating element although changed remain equal to one another. Despite the advantage of a consistent beam location through a range of frequencies, a corporate fed antenna does have the disadvantage of being more complicated than a series fed antenna.
A series fed array antenna consists of a long transmission line having a plurality of radiating elements arranged in series along the transmission line. As a signal travels down the transmission line, a portion of the signal is radiated out of each radiating element in succession. Because each radiating element is spaced at some line length from the other radiating elements, each radiating element will have a certain phase associated with it. The radiating elements each generate a radiation pattern. And because the composite antenna radiation pattern is determined by superimposing the fields of each radiating element, the shape and direction of the antenna radiation pattern is determined by the relative phases and amplitudes of the currents at the individual radiating elements. By properly varying the relative phases, it is possible to steer the direction of the radiation. If the frequency of a signal is changed for one reason or another, the phase at each radiating element will necessarily change. When a frequency of the signals traveling through the array is chosen such that the same phase is applied at all radiating elements, the relative phase difference between adjacent elements is zero and the position of the main beam will be broadside to the array. When the phase applied to the radiating elements are not identical such that the relative phase difference between elements is some value other than zero, the radiation pattern will change and the beam will point in a direction other than broadside. This condition is called squint.
It is known to undo squint through the use of a microprocessor. However, the hardware needed for processing out the squint is expensive. Thus, a means is needed in a series fed antenna for preventing squint. This means should be relatively inexpensive and should preclude the need for a microprocessor.